Recently, wireless communication technology has been often applied to a communication-processing art such as a cellular phone and an information-processing art such as a wireless mouse and an access point, accompanying with development of technology on a semiconductor integrated circuit. As an applied example of such the wireless communication technology, a tag and reader system has been invented. This tag and reader system performs the wireless communication of prescribed data according to the back-scattering scheme and is applied to a system for reading an electronic price tag attached to a dish or a plate in an eatery, for example.
The tag and reader system 1 shown in FIG. 1 has a tag 10 and a tag reader 20′, in which the tag reader 20′ transmits a carrier wave signal having a prescribed frequency to the tag 10 in succession and receives an amplitude modulation signal scattered and returned from the tag 10 to acquire data that is unique to the corresponding tag.
In FIG. 1, the tag 10 receives a carrier wave signal (an interrogation signal) having a frequency of 2.45 GHz and performs, for example, amplitude modulation on the corresponding carrier wave signal based on such the unique data, to transmit a tag amplitude modulation signal thus modulated (hereinafter referred to as “response signal Sf (D)”). This tag 10 is attached to, for example, an object to be specified such as a dish or a plate in an eatery. The tag 10 has a receiving antenna body 1A, a transmitting antenna body 1B, an amplitude modulation unit 2, a memory unit 3, a clock oscillator 4 and a power-supplying unit 5. Although the antenna bodies 1A and 1B are separately described on their operations in two parts, they actually constitute one antenna.
The antenna body 1A receives a carrier wave signal Sf that is the interrogation signal in the tag and reader system 1. As the antenna bodies 1A and 1B, a loop antenna in which conductor is coiled is used. The antenna bodies 1A and 1B are connected to the power-supplying unit 5 that operates so as to supply to the amplitude modulation unit 2, the memory unit 3, and the clock oscillator 4 induced power according to the carrier wave signal Sf received by the antenna body 1A.
The memory unit 3 stores data, which is unique to the object to be specified, such as a price of cooked food on the dish or the plate, and this data is read out thereof based on the clock signal (CLK) to transmit the data to the amplitude modulation unit 2. As the memory unit 3, a read only memory (ROM) is used. The memory unit 3 is connected to the clock oscillator 4 that operates so as to oscillate a clock signal having a prescribed frequency and transmit the clock signal to the memory unit 3. The amplitude modulation unit 2 performs amplitude modulation on the carrier wave signal Sf based on the data read out of the memory unit 3. The amplitude modulation unit 2 is connected to the antenna body 1B from which the response signal Sf (D) thus amplitude-modified is transmitted.
The tag and reader system 1 is also provided with the tag reader 20′ in addition to the above tag 10. The tag reader 20′ operates to transmit the carrier wave signal Sf to the tag 10 and to receive the response signal Sf (D) returned from the tag 10 and process the signal. Actually, the antenna 13B may receive a non-modulated carrier wave composite signal Sf′ that is reflected by and returned from surrounding matters, in addition to the response signal Sf (D). In other word, it may receive a response composite signal Sin including the response signal Sf (D) and the carrier wave composite signal Sf′.
In the tag reader 20′, its main portion includes an oscillator 11, a signal-transmitting unit 12, a transmitting antenna body 13A, a receiving antenna body 13B, a signal-receiving unit 14′ and the like. Although the antenna bodies 13A and 13B are separately described on their operations in two parts, they actually constitute one antenna. The oscillator 11 oscillates a carrier wave signal Sf having a frequency of 2.45 GHz. The oscillator 11 is connected to the signal-transmitting unit 12 that amplifies the carrier wave signal Sf based on an output permission signal S1 received via a control terminal 72 and transmits a carrier wave signal Sf (namely, Sout) thus amplified to the transmitting antenna body 13A. The transmitting antenna body 13A radiates the carrier wave signal Sf thus amplified.
The signal-receiving unit 14′ operates to receive the response signal Sin in receiving it and to perform data de-modulation on it. The signal-receiving unit 14′ has a de-modulation circuit 40 and a data-reading unit 50. The de-modulation circuit 40 is connected to the antenna body 13B and de-modulates the response signal Sf (D) based on the carrier wave signal Sf to transmit data (DATA) that is unique to the tag. The data-reading unit 50 is connected to the de-modulation circuit 40 and operates to read the data (DATA) that is unique to the tag and to transmit it to an output terminal 60. To the output terminal 60, a monitor 16 shown in FIG. 2 is connected through a control device, not shown. The monitor 16 displays the data, which has been read in the data-reading unit 50 and is unique to the tag.
Next, the following will describe some disadvantages in the tag and reader system 1. In the tag and reader system 1 shown in FIG. 2, a tag reader body 101 is provided with the monitor 16, read operation buttons 171, and the like.
In the tag and reader system 1, if an operator pushes any of the read operation buttons 171 of the tag reader body 101 down, the antenna body 13A shown in FIG. 1 radiates the amplified carrier wave signal Sout (namely Sf) so that the carrier wave signal Sf can be transmitted to the tag 10 via a channel I. If an object 90 stays in anywhere around, the carrier wave signal Sf transmitted to the tag 10 is reflected by the object 90 via a channel II and the tag reader 20′ then receives a carrier wave composite signal Sf′ thus reflected. It is to be noted that based on the data, the tag 10 performs amplitude modulation on the carrier wave signal Sf transmitted via the channel I.
On the other hand, the tag reader body 101 receives the response composite signal Sin returned from the tag 10 and performs signal-processing on it. Actually, the response composite signal Sin includes the response signal Sf (D), which is returned from the tag 10 via the channel III, based on the carrier wave signal Sf and the non-modulated carrier wave composite signal Sf′, which is reflected by and returned from the object 90, and the antenna body 13B shown in FIG. 1 then receives the response composite signal Sf′. Thus, if the object 90 stays in anywhere around, the carrier wave composite signal Sf′ returned from the object 90 causes any noise to be generated.
Regarding the tag and reader system, a Japanese patent Application Publication 1 (Japanese patent Application Publication No. H11-239078) discloses a wireless communication system of a modulation-back-scattering scheme. The wireless communication system has an interrogator and remote tags and the interrogator transmits an interrogate signal having a prescribed frequency to any of the remote tags. In this moment, as the interrogate signal, a narrow band downlink signal is used. Further, the remote tag performs an amplitude modulation on it to generate a response signal that has become a wide band uplink signal after the amplitude modulation and the interrogator receives the response signal to process it. Thus, use of the narrow band downlink signal and the wide band uplink signal allows a modulation-back-scattering (MBS) wireless communication system having a processed gain concerning an MBS background noise to be provided.
Regarding a background noise reduction method in such the system, a Japanese Patent Application Publication 2 (Japanese Patent Application Publication No. H07-193519) discloses a background noise reduction apparatus. This background noise reduction apparatus has a de-modulation unit, a frame power measurement circuit, a linear prediction analysis circuit, an inverse filtering circuit, and a subtraction unit. The frame power measurement circuit receives a de-modulated audio signal (hereinafter referred to as “de-modulation signal”) from the de-modulation unit and obtains a power level thereof for every frame to compare it with a previously set threshold value. Based on a comparison result thereof, if the power level indicates the one on or below the threshold value, the linear prediction analysis circuit receives the de-modulation signal and performs linear prediction analysis on it to generate a linear prediction coefficient. The inverse filtering circuit performs an inverse filtering process on the de-modulated signal based on the linear prediction coefficient to obtain a prediction value thereof. The subtraction unit subtracts the prediction value from the received de-modulation signal. This allows only the background noise level to be decreased below the previously set value so that a receiver side can realize comfortable communication with the background noise being used as a part of the information.
The tag and reader system to which the MBS wireless communication system is applied relative to the conventional example has some disadvantages as follows:
(i) As shown in FIG. 2, if the object 90 stays in anywhere around the tag reader body 101, the carrier wave composite signal Sf′ reflected by and returned from the object 90 causes any noise. This may result in a deterioration of S/N ratio of the response signal from the tag 10; and
(ii) In order to restrain the deterioration of S/N ratio of the response signal received from the tag 10, such method is conceivable that the wireless communication system disclosed in the Patent Application Publication 1 and the background noise reduction apparatus disclosed in the Patent Application Publication 2 are combined. If, however, such the two technology concepts are merely combined, it is difficult to teach a configuration for eliminate the carrier wave composite signal Sf′ reflected by and returned from the object 90. Therefore, a merely combined system accompanies a difficulty to compensate a data modulation component in the proper response signal transmitted from the tag 10.